The present invention relates to a process for processing a thermoplastic, enabling highly transparent articles, in particular films, to be manufactured. It also relates to special films which can be obtained by this process.
Transparent films constitute a very extensive application of thermoplastics, especially in the field of packaging. In particular, by way of example, among the polymers used for manufacturing transparent films, propylene polymers occupy an increasing place.
Improving the transparency of films, which remains a constant objective of the industries in question, may not only be achieved by optimizing the chemical nature of the polymers but also by the choice of particular processing conditions. As a general rule, the transparency of semicrystalline polymers may be increased by reducing their crystallinity and/or the size of their crystallites.
With regard to the chemical nature of the compositions used, techniques known for this purpose are in particular the addition of a comonomer so as to obtain a less crystalline random copolymer, or the addition of a nucleating agent (talc, etc.) which reduces the size of the crystallites and increases their number. With regard to the processing conditions, conventional techniques consist in particular in drawing the film in the solid state so as to break up the crystallites (for example, in the case of biaxially oriented films), or else by very rapidly cooling the films (quenching) immediately after their manufacture, so as to reduce the size of the crystallites and simultaneously to reduce the crystal-linity of the polymer. However, in the latter case, the crystallinity will tend to increase over time, and will do so all the more rapidly if the film is reheated.
Several methods have already been proposed for the purpose of improving certain mechanical and optical properties of thermoplastic-based films. Thus, in the document FR 1,211,161, such a film, manufactured beforehand, is reheated above its melting point (Tf), then cooled between Tfxe2x88x9212xc2x0 C. and Tf, then drawn at least 150% at this temperature, and finally cooled to at least 75xc2x0 C. under tension.
A drawback of this process is that it requires a separate step to manufacture the film, followed by a reheating step, this being highly disadvantageous within the framework of industrial exploitation.
The present invention therefore aims to provide a process which is simple to employ and which makes it particularly possible to manufacture flat thermoplastic-based articles having improved optical properties, in particular transparency.
More specifically, the invention relates to a process for processing a semicrystalline thermoplastic, which includes, in succession, the following steps:
(1) a molten semicrystalllne thermoplastic extrudate is cooled to a temperature T1 which is greater than the crystallization temperature (Tc) of the thermoplastic and less than Tf+5xc2x0 C., Tf denoting the melting point of the thermoplastic;
(2) next, the extrudate is drawn in the molten state;
(3) next, the extrudate is relaxed, at a temperature T3 which is between Tc and Tf, for a duration of at least equal to the average relaxation time (xcfx843) of the thermoplastic at the relaxation temperature and after the drawing;
(4) next, the extrudate is quenched to a temperature T4 which is less than Tc.
This process makes it possible to manufacture flat articles such as films (having a typical thickness of the order of a hundred xcexcm) or sheets (having a typical thickness of the order of a millimetre). For reasons of simplicity, the specific details of the process of the invention will essentially be described within the context of the manufacture of such flat articles, although this process also makes it possible to manufacture any other article of constant cross-section, such as a bar, profile, pipe, etc., by means of a few modifications obvious to those skilled in the art.
Conventionally, the prior step of melting the thermoplastic takes place at a temperature well above its melting point (Tf). Such prior melting may be carried out by means of any known device, such as an extruder or mixer. The mass of thermoplastic produced, generally continuously, by this device will be termed the extrudate, even if an extruder is not involved.
Compared with the prior known process mentioned above, the process thus proposed is simpler given that it is carried out starting from a molten thermoplastic and does not require a prior step of forming the article and a subsequent step of reheating the latter. The proposed process is furthermore distinguished by the presence of a high-temperature relaxation step (3) followed by a short and vigorous cooling (4), whereas, according to the previously known process, the film is simply left to cool naturally, and under tension.
It will be recalled that the melting point (Tf) and crystallization temperature (Tc) correspond to the endothermic and exothermic peaks measured by differential scanning calorimetry (DSC), respectively during heating and cooling, of the thermoplastic in question, in the absence of any external stress.
The thermoplastic is semicrystalline, that is to say that it comprises at least 50% by weight of one or more semicrystalline thermoplastic polymers, such as, for example, polyolefins or polyvinylidene fluoride. It may furthermore include a minor proportion of one or more other thermoplastic polymers, as well as, optionally, modest amounts of additives such as antioxidants, stabilizers, pigments, etc. Preferably, the thermoplastic includes at least one polyolefin. More particularly preferably, the thermoplastic material includes at least 50% by mass of one or more polyolefins. Ideally, the thermoplastic essentially consists of one or more polyolefins. A propylene polymer may advantageously be used as the polyolefin. Excellent results have been obtained with ethylene-propylene copolymers, as well as with propylene homopolymers.
The cooling (1) of the extrudate preferably takes place at a temperature T1 less than Tf this cooling may be carried out in any known manner, for instance by contact with at least one chill roll placed in the immediate vicinity of the device from which the extrudate leaves. This roll may, for example, be cooled by the internal circulation of a fluid maintained at the appropriate temperature by a thermal conditioning device. In order to reduce the cooling time, it is advantageous to cool both sides of the extrudate, for example on two consecutive conditioned rolls, the extrudate describing an S-shaped path. Instead of a single roll, it is also possible to use a pair of rolls pinching the extrudate, when this is in the form of a strip. The drawing (2) may be carried out in any known manner, for instance by exerting tension on the extrudate by means of a roll or a pair of rolls, this or these being placed downstream and driven by a motor whose speed is judiciously controlled. Good results are obtained if the draw ratio during step (2) is at least 200% and preferably at least 300%. When at least one roll is used for each of the cooling (1) and drawing (2) steps, the draw ratio is approximately the ratio of the peripheral velocities of the roll or rolls used for the cooling (1) and of the roll or rolls used for the drawing (2).
The drawing conditions are advantageously such that the velocity gradient is at least 1/(20xcfx840) where xcfx840 denotes the average relaxation time of the thermoplastic at the extrusion temperature (T0) in the absence of any drawing stress. Preferably, this velocity gradient is at least 1/(10xcfx840). The relaxation time xcfx840 is defined by the       τ    0    =            2      ⁢              xe2x80x83            ⁢              η        0            ⁢              M        c                    ρ      ⁢              xe2x80x83            ⁢              RT        0            
in which xcex70 denotes the dynamic viscosity of the thermoplastic for velocity gradients tending towards 0, xcfx81 is its density, Mc is the critical (weight-average) molecular mass above which xcfx810 is proportional to the power 3.4 of the molecular mass, and R and T0 denote the ideal gas constant and the absolute extrusion temperature (at the die exit).
The drawing may be uniaxial or biaxial; however, it is preferably uniaxial, that is to say that the extrudate more or less maintains its initial width, a slight transverse necking being, however, possible.
The region where the extrudate is undergoing this drawing is advantageously protected from undesirable temperature variations, in particular draughts; optionally, this region is equipped with heating means, for example infrared heating means, enabling the thermoplastic to be maintained at the required temperature.
An important characteristic of the process of the invention is that the drawing (2) takes place while the thermoplastic is still in the molten state. This does not necessarily mean that the temperature T2 at which the drawing takes place is greater than Tf. This is because, within the temperature range lying between Tc and Tf, a semicrystalline thermoplastic may indifferently be in the molten or solid state, depending on its thermal history. In general, within this range, a semicrystalline thermoplastic will be in the molten state if it has been cooled from a temperature above Tf, and in the solid state if it has been heated from a temperature below Tc. In the process of the present invention, it is the first of these two cases which is adopted; the drawing generally takes place at a temperature (T2) lying between Tc and Tf+5xc2x0 C., preferably less than Tf.
According to a very simple variant of the process, the drawing takes place approximately at the temperature T1 of the cooling (1), without either additional cooling or reheating.
When it is carried out under the conditions indicated, the drawing causes very extensive crystalline nucleation of the thermoplastic, greater by several orders, in terms of the number of crystallites, than heterogeneous nucleation (by means of talc, etc.).
Furthermore, the drawing conditions are such that no inversion of Tc and Tf occurs, contrary to what is generally the case when a thermoplastic is biaxially oriented in an extrusion die, when it is subjected to enormous stresses.
Any relaxation of a thermoplastic is intended in general to reduce its internal tensile stresses, in particular those of its amorphous phase, and thus the extent of any subsequent shrinkage in the case of reheating. However, within the scope of the process of the present invention, a relaxation seems a priori to have little to recommend it, given that there is a risk of it causing the benefit of the drawing step (2) to be lost and thus the transparency of the manufactured articles to be reduced.
Surprisingly, however, it has been found that the specific relaxation conditions mentioned above enabled the benefit of the drawing step (2) to be maintained while still reducing the magnitude of any subsequent shrinkage of the articles during their use.
Preferably, the relaxation (3) takes place at a temperature T3 greater than Tc+5xc2x0 C. Advantageously, it takes place at a temperature less than Tfxe2x88x925xc2x0 C. The duration of the relaxation may be chosen depending on the desired rigidity of the article manufactured: a longer duration of relaxation will lead to a lower rigidity. Preferably, this duration is at least 2xcfx843. The value of xcfx843 is calculated as indicated above, by replacing T0 by T3 (absolute temperature) (therefore neglecting any variations in xcex70 and in xcfx81). Moreover, the duration of the relaxation is advantageously at least 1 s, preferably at least 2 s.
Preferably, the relaxation takes place at constant dimensions and in the absence of any external stress. In the case of the manufacture of flat articles, it may, for example, take place on the surface of a roll maintained at the appropriate temperature, located downstream of the drawing device (for example, the drawing roll). According to an advantageous variant, the relaxation takes place on the roll which exerts a tensile force on the extrudate in order for it to undergo drawing (2), which makes it possible to save on one roll. In this case, it is inevitable that a certain portion of the extrudate in contact with the roll, lying on the xe2x80x9cupstreamxe2x80x9d side, i.e. in the direction of the device from which the extrudate leaves, can slip with respect to the roll and thus be subjected to drawing stresses. The length of this portion is generally less than 1 cm. On the other hand, that part of the extrudate which lies downstream of this portion is not subjected to any drawing stress; under the usual industrial conditions, its length is generally at least 20 cm. It is preferable to ensure that the total length of extrudate in contact with the drawing roll is sufficiently great so that only a small proportion of the extrudate in contact with this roll is subjected to drawing stresses. If a particularly long duration of relaxation is desired, it is possible to use a larger diameter roll or to use several consecutive rolls.
It should be noted that such a relaxation cannot be achieved inside an extrusion die because of the high shear stresses which the thermoplastic would be subjected to therein.
An advantageous variant of the process of the invention consists in combining with it, prior to the relaxation step (3), a step of embossing the extrudate, in which the latter is pinched between two metal rolls, preferably having a high hardness (a Vickers hardness preferably greater than 150 HV), for example consisting of or coated with steel or cast iron. At least one of these rolls must be sand-blasted or etched on at least part of its surface so as to produce the embossing in question. The type of embossing of each side of the extrudate may differ: for example, it is possible to use a first sandblasted roll (leading to the formation of a matt side) and a second roll etched with a regular pattern. The fact of not using a pressure roll consisting of or coated with an elastic material offers several advantages, in particular with regard to the quality of the simultaneous embossing of both sides of the extrudate, the possibility of precisely controlling the thickness of the extrudate, the absence of any risk of the extrudate sticking to the rolls, and the high and precise temperature which may be given to each of the two rolls, which contributes to the quality of the embossing. It is preferable for both rolls to be temperature conditioned (to identical or different temperatures), preferably to a temperature of between Tcxe2x88x925xc2x0 C. and Tfxe2x88x925xc2x0 C. This variant is particularly advantageous for the treatment of high-molecular-mass polyolefins (MFI less than 10 g/10 min, preferably less than 3 g/10 min, measured according to the ASTM D1238 standard, under a load of 2.16 kg at 190xc2x0 C. for polymers containing at least 50% by weight of ethylene and at 230xc2x0 C. for those containing at least 50% of propylene). Extrudates essentially consisting of these high-molecular-mass polyolefins are not generally embossable using the known processes, in particular because of their excessively high elasticity which leads to problems of thickness uniformity and embossment uniformity. They lend themselves particularly well to the production of articles for medical use, such as blood bags, fluid bags, bandages, etc. In these applications, they have the advantage of being sterilizable (at high temperature), without running the risk of them sticking to each other because of the surface embossment; furthermore, they are anti-blocking when cold.
Further details relating to the realization of this variant may be found in Belgium Patent Application No. 9500436 (EP 96201237.3) (Solvay).
This embossing step may be carried out at any moment between the extrusion of the extrudate and the relaxation step (3); however, for the purpose of not disturbing the embossment produced, it is preferable that at least 70% of the total drawing which the extrudate undergoes during the execution of the process of the invention take place before the embossing step. Particularly preferably, the embossing takes place between the drawing step (2) and the relaxation step (3). According to an advantageous subvariant, the relaxation (3) takes place at the surface of a roll which is one of the two embossing rolls.
In the 4th step (quenching), a second, very rapid, cooling further contributes to increasing the transparency. The quenching (4) consists of a sudden cooling, which takes place at a temperature T4 of less than Tc, preferably less than Tcxe2x88x9250xc2x0 C., and particularly preferably less than Tcxe2x88x9280xc2x0 C. It is desirable for the duration of this cooling to be less than 3 s, preferably less than 1 s.
This quenching may especially be carried out by making the extrudate pass through a thermostatted water bath or over a chill roll. Simple air cooling to ambient temperature, generally applied in blown-film manufacturing processes, is not sufficiently rapid. Consequently, contrary to the present process, these processes lead to mediocre optical properties.
The rolls optionally used in the process of the invention preferably have a good surface finish. Advantageously, they are chromium-plated metal rolls.
The fact that the steps described above take place xe2x80x9cin successionxe2x80x9d, does not mean that they are immediately consecutive. In other words, one or more other steps may be incorporated into the process described above, as long as they do not fundamentally affect the effects, this being applicable equally well before the above-mentioned steps, after them or between some of them. For example, if the extrudate is deposited on a chill roll (step 1) after it leaves the flat film die of an extruder, it is conventional to adjust the speed of rotation of this roll to a value such that it exerts a certain tension on the extrudate, so as to prevent the latter from creeping excessively. Since this xe2x80x9cpredrawingxe2x80x9d takes place at a relatively high temperature, it has little effect on the orientation, the crystallinity etc., of the thermoplastic, contrary to the drawing step (2) described above, which takes place at a lower temperature.
Likewise, in the case of the manufacture of a film, when the film is wound up after the quenching (4), the tension exerted on the film by this winding may have repercussions upstream of the quenching step (4) and also cause slight drawing of the film between steps (3) and (4), because the film is still at a high temperature. This possible slight drawing does not significantly affect the optical properties of the film. Nor does it significantly affect its mechanical properties (rigidity, etc.), as long as it does not exceed approximately 25%.
On the other hand, the term xe2x80x9cin successionxe2x80x9d does mean that the abovementioned steps are essentially separate in time and do not overlap. In particular, the cooling (1) and the drawing (2) do not take place simultaneously, contrary to what occurs in blown-film manufacturing processes.
The process of the invention gives excellent results when it is applied to the manufacture of a flat article, for example a film.
Although films based on homopolymer polypropylene (PP) are markedly less transparent than those based on propylene copolymers, at least when they are produced in a conventional manner, the process described above makes it possible, in particular, to manufacture films of homopolymer PP, with a thickness of approximately 200 xcexcm, having a haze (measured according to the ASTM D 1003-95 standard) of less than or equal to 3% and a total transparency with respect to air (measured according to the ASTM D 1746-95 standard) of greater than 70%. Such optical properties are quite exceptional for a homopolymer-based film and can be compared favourably with copolymer-based films.
This is why another subject of the present invention is a film essentially consisting of at least one propylene homopolymer, with a thickness of from 100 to 250 xcexcm, having a hot shrinkage of at most 20%, a haze of less than or equal to 3% and a total transparency with respect to air of greater than 70%, which is capable of being manufactured by means of the abovementioned process. The hot shrinkage denotes the shrinkage measured after a period of 30 minutes in a ventilated oven at a temperature of Tfxe2x88x9225xc2x0 C. The presence of a limitation regarding the shrinkage is due to the fact that, among all known homopolymer-PP-based films, those which are (biaxially) oriented using a conventional technique, i.e. in the solid state, may indeed have similar optical properties, but their hot shrinkage (defined as being (L0xe2x88x92L1)/L1, where L0 and L1 respectively denote the length before and after shrinkage) is then considerable, often very significantly greater than 100%.
The film thus defined includes at least 95% by weight of at least one propylene homopolymer with respect to the total weight of the thermoplastics, and optionally one or more other conventional polymers and/or additives. However, it is noteworthy that the abovementioned optical properties can be obtained, in particular, when the film is free of nucleating additives, such as talc, etc.
The film thus defined comprises a single layer; however, it may, a of course, be combined, using conventional techniques such as, for example, laminating or extrusion coating, with one or more other layers of thermoplastic and/or also with substrates of another kind.
According to an advantageous variant of the invention, a thin sealing layer is associated with the films obtained according to the process described above. This is because, in some applications, such as the manufacture of bags intended to contain blood or other liquids for medical use, it is necessary to weld two films together easily and reliably, i.e. without causing local degradation of the mechanical performance thereof, and to produce a strong weld. In particular, liquid-containing bags for medical use must generally be able to withstand being sterilized and possibly being dropped after they have been filled, without the strength of the welds being affected. When such articles are manufactured from monolayer films, it is observed that high-temperature sterilization greatly reduces the strength, especially the impact strength of the welds.
It has been found that this problem could be solved by using a sealing layer having specific properties.
This is why an advantageous variant of the present invention relates to a process as described above, applied to the manufacture of a multilayer film, in which the extrudate subjected to the abovementioned steps includes at least two layers, one forming the main layer (A) of the film and another, external layer being a sealing layer (B) which has a thickness of less than that of the main layer (A) and the thermoplastic of which it is composed has a melting point and a melt flow index which are respectively less than and at least twice as great as those of the thermoplastic of which the main layer (A) is composed. This variant may be produced by any known technique, for example by using a coextrusion die fed by at least two separate extruders.
Typically, the sealing layer (B) has a thickness equal to at least 5% of that of the main layer (A). Moreover, its thickness generally is less than 40% and preferably less than 20% of that of the main layer (A). In absolute values, it is preferred for the thickness of the sealing layer to be at least 5 xcexcm, in particular at least 20 xcexcm.
The thermoplastic of which the main layer (A) is composed satisfies on all points the definition of the thermoplastic given in the general definition of the process, including with regard to its preferred variants. It should be noted that in the present variant the various parameters (Tc, Tf, etc.) mentioned in the said general definition refer specifically to the thermoplastic of which the main layer (A) is composed.
Another simple variant consists of a process as defined above, applied to the manufacture of a multilayer film, furthermore including an extrusion-coating or laminating step, after the quenching step (4), by which the film obtained (A) is provided with a sealing layer (B), the thermoplastic of which it is composed having a melting point and a melt flow index which are respectively less than and at least twice as great as those of the thermoplastic of which the main layer (A) is composed.
The melt flow indices are measured according to the ISO R1133 standard (1981 version), under the usual conditions depending on the materials in question.
Any thermoplastic satisfying the abovementioned conditions can be used as a constituent of the sealing layer (B). Preferably, this is a polymer or polymer blend compatible with the main layer (A), thereby making it possible to avoid using an adhesive interlayer. Good results have been obtained when the thermoplastic of which the sealing layer (B) is composed includes at least one xcex1-olefin copolymer, in particular an ethylene-propylene.(E/P) copolymer, preferably an E/P copolymer with proportions by weight of from 1/99 to 8/92.
The thermal properties of the sealing layer (B) ensure that only the latter layer melts during welding; the operation is more rapid and the welds have greater impact strength. Advantageously, the melting point of the thermoplastic of which the sealing layer (B) is composed is at least 10xc2x0 C., and preferably at least 20xc2x0 C., less than that of the thermoplastic of which the main layer (A) is composed. Advantageously, the melt flow index of the thermoplastic of which the sealing layer (B) is composed is at least twice as great, preferably at least four times as great, as that of the thermoplastic of which the main layer (A) is composed (under the abovementioned measurement conditions).
If necessary, the flexibility of the resulting multilayer film may be increased when the thermoplastic of which the main layer (A) is composed and/or the thermoplastic of which the sealing layer (B) is composed include at least one copolymer which is compatible with its other constituent or constituents and which is capable of increasing the flexibility of this layer (B). Preferably, at least one block copolymer of the styrene-ethylene-butadiene-styrene (SEBS) type (for example, the product KRATON(copyright) 1657 from Shell) or of the styrene-ethylene-propylene-styrene (SEPS) type is used. Advantageously, the weight content of this copolymer within the layer (B) is from 10 to 40%.
In addition to the main layer and the sealing layer, the film may optionally also include one or more other layers based on one or more thermoplastics.